In the electrophotographic method, after developing an electrostatic latent image formed on a latent image carrier provided with a photoconductive substance using toner particles containing a colorant, the electrostatic latent image is transferred on an intermediate transfer medium, and then the thus-obtained toner image is transferred on a recording material such as a paper to be fixed by heat, pressure, and the like, thereby forming a copy or a printed material. Since the toner remaining on the latent image carrier after the transfer process can be the cause of unwanted reverse printing and photographic fogging on the recording material in an electrophotographic process which is a post process, a cleaning member is used for removing the residual toner on the latent image carrier.
A so-called blade cleaning method of scraping the residual toner by abutting a urethane blade or the like to the latent image carrier after the transfer process is widely used for the cleaning member. However, since the cleaning member employing the blade cleaning method causes a film of the latent image carrier to be shaved, such cleaning member has a problem of reduced latent image carrier life. Also, since the shaved film of the latent image carrier is subject to a fluctuation in electrostatic capacity of the latent image carrier, a fluctuation in image forming condition in the electrophotographic method is caused to raise a problem of a deterioration of image quality. Further, since the cleaning member occupies a space around the latent image carrier for its installation, the cleaning member is not satisfactory for coping with downsizing of the latent image carrier which has recently been of an increased demand.
Accordingly, the image forming apparatus of the cleanerless method based on a so-called simultaneous development and cleaning, which is capable of collecting the toner left after the transfer into a developer by setting a potential difference during the development, has been proposed (References 1 to 3). Though the image forming apparatus of cleanerless method is downsized, since the residual toner, contaminants, and paper dust on the latent image carrier are collected in the image forming apparatus, problems of instable charging characteristics of developing agent, mixing of colors due to a reduction in transfer efficiency, a generation of fogging, and reverse transfer toner, and an insufficient color reproducibility have been raised.
A corona charger, for instance, is used for uniformly charging a surface of the latent image carrier, and such corona charger is subject to discharge product such as NOx and ozone due to its discharge. NOx reacts with moisture in the air or the like to be nitric acid or reacts with metals to generate a nitrate salt. The acid and salt has a property of depositing on the surface of the latent image charier in the form of a thin film and reduces a resistance of the latent image carrier surface to cause disturbances in electrostatic latent image to be formed on the latent image carrier surface, which results in disturbed image. Therefore, Reference 3 discloses the use of a contact member of a development roller, a transfer roller, or the like of a developer as a polishing member, but such usage raises a problem of abrasion deterioration of the latent image carrier. Also, in Reference 4, the disturbance in electrostatic latent image on the latent image carrier surface is suppressed by preventing influences by moisture in the air by blowing dehumidified air around the latent image carrier, but this does not solve the problem caused by the collection of the residual toner, the contaminants, and the paper dust on the latent image carrier into the developer in the simultaneous developing and cleaning.
Also, a method of cleaning in the intermediate transfer medium by: using a spherical toner for the purpose of a high transfer efficiency; setting a sphericity of the toner to 0.96 or more; and collecting the residual toner on the latent image carrier followed by removing the residual toner to the intermediate transfer medium (Reference 5) have been proposed, but this method is problematic from the viewpoint of downsizing of the latent image carrier though this method is capable of preventing the color mixing of toners thanks to the use of a retention roller.
In Reference 6, the simultaneous development and cleaning by using a spherical toner having a sphericity of 0.950 to 0.995 in combination with a magnetic brush development is performed, but this method has the above-described problem caused by the collection of the residual toner, the contaminants, and the paper dust on the latent image carrier to the developer.
The inventors of this invention have proved that it is possible to reduce the fogging and to improve the transfer efficiency by setting a work function of the toner to a value larger than those of the latent image carrier and the intermediate transfer medium (Reference 7), but the improvement in transfer efficiency was about 96% and insufficient for realizing the cleanerless method which requires a transfer efficiency of 99% or more. The method did not consider the application thereof to the cleanerless method.
As described in the foregoing, the conventional cleanerless methods do not meet the conflicting demands of downsizing by the use of the cleanerless method and prevention of filming on the latent image carrier surface.
[Reference 1] JP 5-53482 A
[Reference 2] JP 8-146652 A
[Reference 3] JP 10-240004 A
[Reference 4] JP 2004-37899 A
[Reference 5] JP 11-249452 A
[Reference 6] JP 2000-75541 A
[Reference 7] JP 2003-107770 A
An object of this invention is to provide an image forming apparatus of a cleanerless method wherein a member to be brought into contact with a latent image carrier is merely an intermediate transfer medium, the image forming apparatus being downsized and capable of reducing deposit on a surface of the latent image carrier.